Highly Stereoselective C‐Glycosylation by Photocatalytic Decarboxylative Alkynylation on Anomeric Position: A Facile Access to Alkynyl C‐Glycosides

Abstract

The highly stereoselective synthesis of diverse medicinally valuable alkynyl C-glycosides under mild and green reaction conditions remains a great challenge. Herein, we wish to report a visible-light induced photocatalytic decarboxylative alkynylation approach. By utilizing an iridium photocatalyst in the presence of visible light, glycosyl radicals are in-situ generated via the decarboxylation of anomeric acids and efficiently coupled with a variety of ethynylbenziodoxolones (EBXs), which allows the facile synthesis of a variety of alkynyl C-glycosides in moderate to high yields and with excellent diastereoselectivity..

Synthesis of reverse glycosyl fluorides via organophotocatalytic decarboxylative fluorination of uronic acids

“…More importantly, Mes-Acr + -catalyzed decarboxylative fluorination enables the synthesis of RGFs 2r–u from benzyl- or benzoyl-protected gluco- and galactopyranuronic acids 1r–u , typical hexopyranuronic acids which are relatively underexplored. 12,13 We found that protecting groups have paramount influence on the decarboxylative fluorination of hexose-based pyranuronic acids. Specifically, Bn-protected glucuronic acids 1r and 1s with either an α-methoxy or β-substituent at the anomeric position furnished the respective RGFs 2r and 2s in 63% and 72% yields with identical stereoselectivities.…”

“…These unique diastereoselectivities are attributed to the stability of α-oxyalkyl radicals arising from the n–p c interaction between the n orbital of the lone electron pair of endocyclic oxygen and the p orbital of the C-4 radical as well as from the quasi-homo anomeric effect between the σ* orbital of O-3 and C-4 radicals. 12,20 In addition, it should be noted that 1f was converted into 2f in 30% yield with Cs 2 CO 3 as the base instead of CsHCO 3 . This result demonstrated the important role of CsHCO 3 in decarboxylative fluorination of uronic acids bearing protecting groups susceptible to the base.…”

“…10,11 Functionalization of furanos-4-yl or pyranos-5-yl radicals as versatile species has witnessed significant progress in making structurally diverse sugars through C–C and C–heteroatom bond formation. 12 Among these achievements, the MacMillan group reported photocatalyzed radical decarboxylative fluorination of ribofuranuronic acid as a single example of the synthesis of RGFs for the first time (Scheme 1). 13 Doyle and co-workers recently described reductive decarboxylative fluorination of redox-active N -hydroxyphthalimide ester derived from ribofuranuronic acid (Scheme 1).…”

Preparation of glycosyl carboxylic acidsviastereoselective synthesis and oxidative cleavage ofC-vinyl glycosides

“…For example, they can be used as coupling reagents for biological probes 2 and for the synthesis of oligosaccharides via multi-component reactions. 3 They are also valuable intermediates employed in C–H activation, 4 radical reactions, 5 and the Liebeskind–Srogl reaction 6 for the synthesis of C -glycosides. 7…”

“…7 b , 19 Recently, glycosyl carboxylic acids have been frequently utilized as radical precursors in decarboxylative C -glycosylation. 5 However, uniformly protected substrates were usually used. With differently O-5 protected furanosyl acids in hand, we thought it was worthwhile to probe the effect of the protecting group, as shown in Zhang's work.…”